1. Field of the Invention
The present invention relates to a magnetic resonance imaging apparatus (to be abbreviated as an MRI apparatus hereinafter) and, more particularly, to an MRI apparatus for imaging a large number of slices or slabs.
2. Description of the Related Art
Generally, an MRI apparatus selectively excites a predetermined region called a slice (two-dimensional region) or a slab (three-dimensional region) at once, adds phase information corresponding to the position of spins to a magnetic resonance signal (MR signal) generated from the whole selective excitation region (this process is called encode), and repetitively acquires the MR signals while changing the encode amount. When the signal acquisition of one region is completed, the apparatus acquires MR signals of an overall region to be imaged while changing the selective excitation region. This imaging method is called a sequential multi-slice imaging method or a sequential multi-slab imaging method. The apparatus then performs reconstruction processing for the acquired MR signals, i.e., performs two-dimensional Fourier transform for each slice, or three-dimensional Fourier transform for each slab, thereby reconstructing a two-dimensional or three-dimensional image.
To efficiently acquire MR signals from a wide region, a so-called multi-slice imaging method or multi-slab imaging method has been developed in which, during a repetition time TR for a certain slice, signals from a large number of other slices or slabs are acquired.
Regardless of whether the method is a simple sequential multi-slice or multi-slab imaging method or a so-called multi-slice or multi-slab imaging method, images are reconstructed in units of selective excited regions (slices or slabs) on the basis of the signals acquired from these excited regions.
It is unfortunate that the above conventional imaging methods have the following drawback. That is, in imaging a sagittal image or a coronal image which is long in the body-axis direction, a difference is produced in the signal intensity between imaging regions neighboring along the body-axis direction. This results in a discontinuous signal intensity near the boundary, leading to a nonuniform image quality. Especially when MR angiography is performed using the multi-slab imaging method or the sequential multi-slab imaging method, the blood flow signal intensity decreases due to a saturation effect on the downstream side of the blood flow in each slab. Consequently, the signal intensity becomes discontinuous in the boundary between the slabs, and the brightness of the blood flow image also becomes discontinuous in the resulting MR angiography accordingly.
On the other hand, a method has been proposed in which signals in a broad range along the direction perpendicular to the slice are efficiently acquired in a manner similar to helical scan of an X-ray CT apparatus which gradually moves the slice position ("Helical Scan for Time Resolved MRI at 0.5 T", V. Rasche et al., Abstract Book of SMRM '93, p. 479).
Unfortunately, this method reconstructs an image by approximately or virtually calculating an MR signal of a virtual slice, and hence does not accurately reflect information in the slice direction.
As described above, the conventional magnetic resonance imaging apparatuses have the problem that it is not possible to obtain a high-quality, uniform image in a broad imaging region under the imaging conditions in which signals cannot be acquired at once from a wide selective excitation region or in which it is better not to widen the selective excitation region.